Door controls of this type are mainly used in public vehicles, for example buses, which serve public transportation. The controls are mostly operated pneumatically or electro-pneumatically, that means, the driver releases respective pneumatic or electro-pneumatic impulses for opening or closing the door, which impulses act upon a door control apparatus comprising a door cylinder operable by compressed air supplied via a door valve designed as a control valve. Thus, the stroke movement of the door cylinder piston initiates the opening and closing movements of the door.
Due to the fact that various operational conditions have to be anticipated, especially unsafe actions by the passengers, the control operations must meet a number of limiting conditions. For example, it is generally required that the closing movement of a pneumatically operated vehicle door be automatically reversed to an opening movement when the closing door is jammed by people or objects. This also is applicable for complicated operating conditions which occur, i.e., during winter months when the kinematics of the door are affected by low temperatures, and when the closing movement of the door may be hindered by snow and ice. Since, during opening movement of the door, there also exists danger that people and objects may be caught therein, there should be additionally provided a device, such as an electro-pneumatic switching apparatus which would simply stop the door, by pressure release of the facility, rather than reverse it, because reverse movement of the door could represent a danger to the next person stepping through the same. Such an electro-pneumatic door control has been proposed in German Patent Application Nos. P 29 24 996.5 and P 29 28 801.5, not published. It became apparent, however, that the plurality of required limiting conditions could not be satisfied by standard electro-pneumatic means and could only be met with relatively high costs involved.